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Figure 7. A shaded relief of the generated DEM
6. CONCLUSIONS AND RECOMMENDATIONS
In this paper, we presented a comprehensive methodology for
DEM generation from scenes captured by high resolution
imaging satellites. The proposed methodology has two main
characteristics. Firstly, the parallel projection has been used as
the mathematical model approximating the relationship between
corresponding scene and ground coordinates. The validity of
this model is attributed to the fact that the imaging geometry of
a narrow angular field of view scanner, moving with a constant
velocity and attitude, resembles a parallel projection. In this
regard, one should mention that the derivation of the parallel
projection parameters does not require the knowledge of the
internal and external characteristics of the imaging system. The
involved parameters can be derived through a minimum of five
ground control points. Secondly, the parallel projection model
can be used to generate normalized scenes (i.e., resampled
imagery according to epipolar geometry). The generation of
normalized scenes is advantageous for DEM generation since it
reduces the search space for conjugate points into 1-D along the
epipolar lines as represented by corresponding rows.
The resampled scenes are manipulated by an interest operator to
extract point primitives with a unique grey value distribution at
their vicinity. These points are then considered as candidates for
the matching process, where the correlation coefficient has been
used to evaluate the degree of similarity between hypothesized
matches within. the involved scenes. Matched points are
projected into the object space using an intersection procedure.
Finally, derived object points undergo an interpolation
procedure to produce a dense DEM over the area in question.
Reported results from real datasets verified the validity of the
developed approach for normalized scene generation, where
almost zero y-parallax is observed between conjugate points.
The derived ground coordinates from the SPOT-1 and SPOT-2
data have been proven to be accurate within half a pixel.
However, the derived ground coordinates from the SPOT-5 data
have shown poorer performance, especially in the planimetric
coordinates. Such inaccuracy can be either attributed to poor
quality of the ground control points and/or problems with the
data acquisition process.
Future research work will be focusing on further investigation
into the SPOT-5 dataset to identify the cause of the poor quality
of the derived ground coordinates. Moreover, we will derive a
quantitative evaluation of the quality of the derived DEM by
comparing it to LIDAR data over the same area.
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